1. Field of the Invention
The present invention relates to a so-called composite thermal transfer sheet constructed of a thermal transfer image-receiving sheet having a receptor layer on a substrate and a thermal transfer film having a heat-fusible ink layer on a base film, peelably bonded to each other, such that the receptor layer of the thermal transfer image-receiving sheet and the heat-fusible ink layer of the thermal transfer film are peelably attached to each other.
The present invention also relates to a thermal transfer image-receiving sheet having a receptor layer on a substrate to be used for the composite thermal transfer sheet.
2. Description of the Related Art
Recently, a thermal transfer medium of a heat-fusible transfer type is often used for output prints of a computer, a processor or the like.
As one example of such a thermal transfer medium of the heat-fusible transfer type, a thermal transfer film of the heat-fusible transfer type is constructed of: a base film composed of a plastic film having a thickness of about 3 to 20 .mu. m! such as a polyester film or a cellophane film; and a heat-fusible ink layer which is coated on the base film and is composed of a mixture of vehicle (e.g. wax) and colorant such as pigment or dye.
When printing is affected on a thermal transfer image-receiving sheet using the aforementioned thermal transfer film, the thermal transfer film is supplied from a roll thereof, while a continuous or cut-sheet-like thermal transfer image-receiving sheet is also supplied, so that the former and the latter are superposed on each other on a platen. Then, in such a state, heat is applied to the thermal transfer film from the back side surface thereof by a thermal head, so that the heat-fusible ink layer of the thermal transfer film is melted and transferred to the thermal transfer image-receiving sheet. Therefore, a desired image is formed on the thermal transfer image-receiving sheet.
However, if the aforementioned thermal transfer film would be applied to a thermal printer used for printing on a heat-sensitive color-forming paper, a problem is posed as followings. Namely, the heat-sensitive color-forming paper develops coloring by itself (i.e. without the thermal transfer film). Therefore, the thermal printer used for printing on a heat-sensitive color-forming paper does not have any means for transporting the thermal transfer film. Thus, the thermal transfer film cannot be used in the aforementioned thermal printer.
In order to solve the aforementioned problem, the composite thermal transfer sheet is proposed. The composite thermal transfer sheet is constructed by peelably bonding a thermal transfer film and a thermal transfer image-receiving sheet. The thermal transfer film is constructed ofa base film and a heat-fusible ink layer formed thereon, and a thermal transfer image-receiving sheet is constructed of, for example, a plain paper, a synthetic paper, a coated paper and so on. Further, the thermal transfer film and the thermal transfer image-receiving sheet are peelably bonded through a temporary adhesive layer formed on the heat-fusible ink layer of the thermal transfer film. Furthermore, in the composite thermal transfer sheet, the thermal transfer film is peeled from the thermal transfer image-receiving sheet after printing, and thus an image is formed on the thermal transfer image-receiving sheet.
Here, the aforementioned composite thermal transfer sheet is produced by two kinds of methods as followings. Namely, in the first method, a heat-fusible ink layer is formed on the base film of the thermal transfer film, and then, the thermal transfer film is bonded to the thermal transfer image-receiving sheet by an adhesive component in the heat-fusible ink layer. On the other hand, in the second method, a temporary adhesive layer, which includes an adhesive component, is formed on the heat-fusible ink layer of the thermal transfer film, and then, the thermal transfer film is bonded to the thermal transfer image-receiving sheet though the temporary adhesive layer.
However, in the aforementioned composite thermal transfer sheet, since the thermal transfer film is bonded to the thermal transfer image-receiving sheet by the adhesive property of the thermal transfer film side, the adhesive property must be given to a layer to transfer an ink to the thermal transfer image-receiving sheet by heating (i.e. the heat-fusible ink layer). Therefore, there is a problem that an ink of the heat-fusible ink layer is easily transferred to the thermal transfer image-receiving sheet when the composite thermal transfer sheet is preserved for a long period, or preserved in a condition of high temperature. Therefore, a preservation environment and a preservation period are considerably restricted.
If the membrane strength of the heat-fusible ink layer is improved, or if the melting point thereof is made higher, the aforementioned problem may be solved. However, in this case, another problem is posed. Namely, in this case, the ability to transfer the ink of the heat-fusible ink layer to the thermal transfer image-receiving sheet is reduced. Therefore, the thermal transfer image-receiving sheet which can be used for the composite thermal transfer sheet may be restricted, and a printer having high energy with respect to printing is required.
On the other hand, as the usage of printed matter is broadened for various purposes, it is required that the composite thermal transfer sheets constructed of various kinds of the thermal transfer image-receiving sheets and various kinds of the thermal transfer films. Thus, it is necessary that an ink receptor layer is formed on a substrate of the thermal transfer image-receiving sheet, in order to improve the ability of ink reception and in order to improve the ability of paratripsis of printed matter. Further, as polychrome printed matter is developed, it is necessary that a layer to give coloration to the thermal transfer image-receiving sheet is formed on the composite thermal transfer film.